1. Field of the Invention
The invention relates to the encoding, transmission and decoding of graphical image data. In particular, it relates to a novel method and apparatus for combining the techniques of differential pulse code modulation (DPCM) and vector quantization, resulting in a significant improvement in performance.
2. Technical Background
The conventional method of encoding graphical image data by differential pulse code modulation (DPCM) takes advantage of the fact that, in general, the states of adjacent pixels of a graphic image are highly correlated. The technique uses previously processed pixels of the image to predict the state of a present pixel and then transmits the value of the difference between the state of the actual pixel and the predicted state of the pixel. Because the differential values are generally much smaller than the original pixel values, these differential values may be transmitted with fewer bits than the original values. U.S. Pat. No. 4,513,426, issued Apr. 23, 1985, to N. S. Jayant, is illustrative of this encoding technique.
Vector quantization encoding is an encoding technique in which a block of pixel values representing a portion, or cell, of an original image is approximated by comparing the cell with a library of predefined cells and selecting one of the library cells that forms a closest match. A vector code identifies the selected library cell. The vector, rather than the original pixel values or the cell, is transmitted to a decoder. An example of this technique is disclosed in U.S. Pat. No. 3,980,809 to W. C. Cook.
Generally, vector encoding, as described above, offers an improved compression of data over the DPCM technique. However, the improvement in compression is often attended by other problems, such as blocky reconstructed images and a requirement for image dependent library cells.
The above described techniques are applied successfully in many areas. However, because of the importance of data transmission, there is a continuing need for further improvements.